Phase-locked loops are widely used in digital electronics, signal telemetry, and communications applications. A typical phase-locked loop may include a phase-frequency detector, a charge pump, and a voltage-controlled oscillator.
Phase-locked loop integrated circuits produce an oscillator frequency output signal. This oscillator frequency output signal either matches an input frequency signal or can be frequency divided to match an input frequency signal. The oscillator frequency output signal relies upon the voltage at an input node of a voltage-controlled oscillator (VCO) of the phase-locked loop. The phase-locked loop is very sensitive to noise at the input node to the VCO. Consequently, a typical phase-locked loop may include a capacitor between the input node to the VCO and ground. In phase-locked loop circuits manufactured using CMOS technologies, the capacitor used to stabilize the voltage at the input node to the VCO may be a gate capacitor. CMOS technologies continually shrink; consequently gate oxide thicknesses are continually decreasing. Gate capacitors may experience some gate current leakage, and this leakage current is sometimes considered negligible. However, as gate capacitors' gate oxide thicknesses decrease, gate leakage current becomes non-negligible. Therefore, a gate capacitor intended to stabilize the input voltage to a VCO may contribute to voltage fluctuation at the input node to the VCO due to gate leakage current. Accordingly, it is desirable to compensate for such current leakage in a phase-locked loop circuit.